The CMA also called for price caps to be introduced to protect those worst hit by the extra costs.

But now the Committee is calling on the Financial Conduct Authority (FCA) to act "swiftly" to protect people.

It also wants the FCA to "redouble its efforts to make switching a simpler process" after the FCA said that simply publishing the premium wouldn't be enough to get consumers to take action.

The Committee said: "Even if many consumers choose to ignore such information, others will not, and the inclusion of such information may motivate firms to make efforts to reduce their loyalty penalty."

But the Committee's report doesn't touch upon energy and telecoms bills where the loyalty premium is also a big problem - and Citizens Advice believes more action is needed.

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The charity found that broadband providers are overcharging loyal users by £110 a year, while it's £220 a year for mobile customers.

Gillian Guy, chief executive of Citizens Advice, said: "While it’s a good first step to call on banks to report on the size of the loyalty penalty, stronger action is needed to stamp out this bad practice.”

A spokesperson for the FCA said: "These are important issues for the FCA and given the wide range of work to protect vulnerable consumers we have ongoing, we welcome the Treasury Select Committee’s report.

"As we continue to do that work, we’ll carefully consider the Committee’s recommendations."

The Committee's report also calls for debt collection letters to be more supportive and less intimidating - and it wants banks to introduce optional spending controls to help vulnerable consumers better manage their money.

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